This invention relates to ultra low density storable cementitious slurries useful for oil and gas well cementing, as well as to a method of cementing an oil or gas well using such slurries. Such cementitious slurries are further useful in the zonal isolation of subsurface formations.
Hydraulic cements are cements that set and develop compressive strength due to a hydration reaction, not drying. Thus, hydraulic cements can set under water. Consequently, hydraulic cements are often used for cementing pipes or casings within a well bore of a subterranean formation for the construction of oil, gas and water wells, as well as other purposes, such as remedial squeeze cementing. In the oil and gas industry, successful cementing of well pipe and casing during oil and gas well completion requires cement slurries to exhibit a pumpable viscosity, fluid loss control, minimized settling of particles and the ability to set within a practical time.
Conventional mixing equipment is relatively complex and expensive since it must permit the addition of water to dry cement powder, homogenization of the mixture, measurement of its density and, if necessary, recirculation of the mixture. Such requirements further dictate the equipment""s configuration. In addition, dry cement bulk transport equipment must usually be pressurized so that solids can be transferred pneumatically at relatively high rates to the cement pump. Once at the drilling site, the pneumatically conveyed cementing solids must be correctly proportioned and mixed with water, as well as other additives to form a pumpable slurry. The slurry must then be tested for the appropriate density to verify correct solids to water ratio. If cementing solids are not correctly proportioned, the quality of the cement diminishes.
It is sometimes necessary to use a lightweight cement when there is a weak subterranean formation. A lightweight cement exerts a lower hydrostatic pressure on the formation than do regular cements. The lightweight cements should be lower in density without sacrificing strength. The density can be lowered in these cements by either adding sealed microballoons or creating a foam by injecting the cement with a gas, such as nitrogen. This foaming operation can be difficult because additional nitrogen equipment is required and the addition of gas must occur onsite which requires that the density of the slurry be carefully measured before use. Methods of measuring the density as the cement is mixed are known. Such methods can be an inaccurate means of cement solids to water ratio verificationxe2x80x94when the density of the cement approaches the density of water, making differentiation between cement and water difficult.
Low density cements have been used previously. For example, these cements have been disclosed in U.S. Pat. Nos. 4,370,166 and 4,234,344. These patents disclose the use of inorganic material in order to lower the ultimate density of the cement. However, the difficulty with these disclosures, and other conventional cementing slurries, is that they set quickly and therefore cannot be made in advance of their use. A delay in the drilling operation delays the cementing job. If personnel and equipment for cementing idly wait onsite until the drilling ends, a delay can increase the cost of the cementing job. A slurry made in advance and stored until needed would allow the personnel and equipment to make the storable slurry, regardless of whether drilling had ceased or even begun.
Storable cementitious slurries have been made in the past. Examples of these attempts are demonstrated in U.S. Pat. Nos. 5,447,197; 5,547,506; and 6,173,778. These patents, however, do not disclose the use of low density slurries in a storable form.
Therefore, there is a need to create an ultra low-density, storable, slurry capable of being used in cementing a well formation.
The storable, low-density, hydraulically-active, cementitious slurry of the invention is suitable for cementing within a subterranean formation for oil or gas wells. The low-density slurry comprises a hydraulically-active cementitious material, a set retarder, a plasticizer, glass or ceramic micro-spheres, and a strengthening agent. Further the slurry is substantially free of a suspension agent. Use of such slurries in oil or gas wells helps to establish zonal isolation within the cemented wellbore of the subsurface formations.
The storable slurry of the present invention allows the density of the cement to be predetermined and accurately measured before the cementitious slurry is shipped to the wellsite. This in turn, can reduce up-front cost for mechanical equipment such as foam generators and associated nitrogen pumps and storage vessels.
Further, this invention establishes an alternate means to accurately mix and place ultra-low density cement systems without the use of conventional continuous mix cementing equipment or foam and its associated equipment.
Further, this invention allows the reduction and/or elimination of typical and auxiliary cement mixing equipment and material on location such as cement pumps, nitrogen tanks and pumps and the personnel necessary to operate this equipment.
Further, the accurate control of the premixed slurry properties eliminates job uncertainties and leads to a significant improvement in QA/QC.
This invention allows the cementing of a subterranean formation using a low density cementitious slurry after it has been stored for a period of time. It further provides a low-density storable cement slurry that can be made at a different location from the job site. This storable slurry can then be transported to the job site before cementing.
The storable, low-density, hydraulically-active, cementitious slurry is made by adding glass or ceramic micro-spheres to a hydraulically-active cementitious material, adding a strengthening agent to the microsphere containing composition, and mixing the resulting composition with water, a set retarder, and a plasticizer to generate a slurry, wherein the set retarder is present in an amount sufficient to allow storage of the slurry wherein the slurry is substantially free of a suspension agent.
In one method of the invention, a subterranean formation for an oil or gas well is cemented with the storable, hydraulically-active, cementitious slurry formulated by mixing together a hydraulically-active cementitious material with a set retarder, plasticizer, glass or ceramic micro-spheres, and strengthening agent, wherein the storable slurry""s characteristics remain within usable parameters during storage. The storable slurry is then stored until required for cementing, at which time it is activated by mixing together an activator with the storable slurry, pumped into the subterranean formation and allowed to set.
In another method of this invention the steps of the method of cementing the subterranean formation are performed at two different locations. In this method the stable, low-density, hydraulically-active, cementitious slurry is formulated at one location, transferred to a second location, activated at the second location, and then pumped into the subterranean formation for cementing.
Additional effects, features and advantages will be apparent in the written description that follows.